Where Are O2S11 And O2S21 Located?

How To Test The Upstream Oxygen Sensor (Dodge 3.9L, 5.2L, 5.9L)

1995-1999 3.9L, 5.2L, and 5.9L Dodge pick ups, vans, and SUVs are equipped with only one upstream oxygen sensor. 2000 and newer vehicles, come equipped with two upstream oxygen sensors.

On your scan tool (when it's in Live Data mode), the pre-catalytic converter oxygen sensors are identified as O2S11 and O2S21.

O2S11 is shorthand for oxygen sensor bank 1 sensor 1.

  1. On the 5.2L and 5.9L V8 engines, bank 1 is the engine bank that houses cylinders 1, 3, 5, and 7. On this side is where you'll find O2S11.
  2. On the 3.9L V6 engine, bank 1 is the one that houses cylinders 1, 3, and 5. On this side is where this O2 sensor will be located.
  3. If your Dodge pick up (van, SUV) was made between 1995 and 1999, this is the only pre-catalytic converter oxygen sensor it'll have.

O2S21 is shorthand for oxygen sensor bank 2 sensor 1.

  1. On the 5.2L and 5.9L V8's, bank 2 is the one that contains the cylinders 2, 4, 6, and 8 and this is the side where you'll find this O2 sensor.
  2. On the 3.9L V6 engine, bank 2 is the one that has cylinders 2, 4, and 6. It's on this side of the engine where you'll find this O2 sensor.
  3. If your Dodge pick up (van, SUV) is 2000 or newer, you'll have this second O2 sensor installed along with O2S11.

In case you're wondering about the downstream O2 sensor (after the converter), these are referred to as sensor 2... as in O2S12 and O2S22.

Testing The Oxygen Sensor

How To Test The Upstream Oxygen Sensor (Dodge 3.9L, 5.2L, 5.9L)

To test the oxygen sensors, the very first thing you'll do is to induce a rich condition.

The fastest and easiest way to induce this rich condition is to spray a little carburetor cleaner spray into the engine (while it's running) thru' a vacuum hose (that has engine vacuum).

When the carb. cleaner spray is consumed (in the engine cylinder), an instant rich condition will be created. The O2 sensor or sensors will react and produce 0.900 Volts on the scan tool.

Alright, this is what you'll need to do:

  1. 1

    Start the engine and let it run for about 15 minutes

    Now, connect your scan tool and get to its Live Date mode (Don't have a scan tool? Need a scan tool? check out my recommendation: Actron CP9580 scan tool).

  2. 2

    Scroll down to the PID's that are labeled O2S11 and O2S21. These are the PID's that will show you the voltages the O2 sensors are creating.

    Remember, you'll only see the O2S21 if your Dodge vehicle is 2000 or newer.

  3. 3

    Now, take a look at the voltage readings for O2S11 and O2S21 on your scan tool.

    If your Dodge pick up (van, SUV) has been running for about 15 minutes, and the 02 sensors are OK, you should see the voltages moving up and down between 0.200 Volts and 0.900 Volts several times every few seconds.

    If the voltages are not moving between 0.100 and 0.900 Volts, don't worry about it just yet... continue on to the next step.

  4. 4

    With the engine running, spray a little carburetor cleaner into a vacuum hose (that has engine vacuum) while you observe your scan tool's display screen.

    You're not going to be able to spray carb. cleaner into the throttle as the engine is running, because if you were to do this, the engine will die as soon as you disconnect the air duct from it (to spray into it).

    If you spray too much, the engine will stall. If this happens to you, just restart the engine and repeat the step and spray less carb. cleaner spray.

  5. 5

    As you spray some short burst of carb. cleaner into the vacuum hose, you should see the voltage numbers of O2S11 and O2S21 immediately spike to 0.800 to 0.900 Volts. And as long as you're spraying, these voltage number should stay there.

  6. 6

    When you stop spraying, the O2 sensors values should come down and within a few seconds, they should start oscillating between 0.100 Volts to 0.900 Volts. If they don't... don't worry about yet.

OK, the test is done, let's take a look at what your test results mean:


TEST RESULT 1: O2S11 and O2S21 voltage numbers spiked to 0.900 Volts when you sprayed carb. spray into the vacuum hose -This tells you that both oxygen sensors are OK at this point in time. They do not need to be replaced, since what is causing the PCM to think they are fried is something else.

For more info on this, go to the section: Oxygen Sensor Codes Keep Coming Back.

TEST RESULT 2: O2S11 voltage numbers DID NOT spike to 0.900 Volts when you sprayed carb. spray into the vacuum hose -This confirms that bank 1 Oxygen Sensor 1 is no longer working. You can replace the oxygen sensor.

TEST RESULT 3: O2S21 voltage numbers DID NOT spike to 0.900 Volts when you sprayed carb. spray into the vacuum hose -This confirms that bank 2 Oxygen Sensor 1 is no longer working. You can replace the oxygen sensor.